1. Field of the Invention
This invention relates to a novel process for the preparation of 4,4'-dihydroxybiphenyl. 4,4'-Dihydroxybiphenyl is a compound useful industrially as a starting monomer for heat-resistant polymers which have drawn attention in recent years. For example, ternary copolyesters containing p-hydroxybenzoic acid, terephthalic acid and 4,4'-dihydroxybiphenyl as their components include a sort of so-called hermotropic liquid crystalline polymers, which have found actual utility as melt-processable heat-resistant materials for tableware which can be heated in a cooking oven.
2. Related Art
As preparation processes for 4,4'-dihydroxybiphenyl, numerous processes have heretofore been proposed as will be described below.
(1) 2,6-Di-tert-butylphenol is subjected to oxidative dimerization, followed by reduction and dealkylation [J. Org. Chem., 34, 1160 (1969); Japanese Patent Laid-Open No. 11238/1976; ibid., 189127/1983; ibid., 23338/1985; U.S. Pat. No. 4,205,187].
(2) Similar to the above process (1). Transalkylation of phenol with an alkylbenzene is effected instead of the dealkylation (Japanese Patent Laid-Open No. 1434/1984; U.S. Pat. No. 4,482,755; ibid., 4,487,978).
(3) Biphenyl is subjected to sulfonation and alkali fusion (Japanese Patent Laid-Open No. 68154/1977; ibid., 112844/1979; ibid., 57728/1981; ibid., 128726/1981; ibid., 18329/1983; ibid., 18330/1983; U.S. Pat. No. 4,243,822; West German Pat. No. 3,204,079).
(4) Biphenyl is halogenated, followed by hydrolysis (Japanese Patent Publication No. 13451/1986; Japanese Patent Laid-Open No. 17304/1980; U.S. Pat. No. 4,475,000).
(5) Phenol is dimerized (U.S. Pat. No. 3,812,445; ibid., 3,813,445).
(6) Phenol is converted into dihydroxybiphenylsulfon, followed by alkali fusion (Japanese Patent Laid-Open No. 36153/1973).
(7) A halogenated phenol is dehalogenated, followed by dimerization (Japanese Patent Laid-Open No. 53631/1981).
4,4'-Dihydroxybiphenyl can be prepared by using any one of the processes listed above. These processes are however hardly considered to be satisfactory fully as industrial preparation processes.
Namely, the processes (1) and (2) include many steps and are hence complex. Moreover, the reagent or catalyst employed in the dealkylation or transalkylation step is costly, and an irksome purification procedure is required after the completion of the reaction because the removal of the catalyst and the like is difficult.
The process (3) has a problem in the treatment of waste water from the alkali fusion and is not desirable from the environmental standpoint. In addition, the process (3) cannot selectively prepare the 4,4'-derivative alone and thus requires a troublesome purification step.
The process (4) cannot selectively form the 4,4'-derivative alone. A heavy metal compound is often used in the hydrolytic step, leading also to difficult purification.
The process (5) uses a strong acid as a catalyst at an elevated temperature of 225.degree. C. or higher. A reactor of a special material is hence needed.
Likewise the process (3), the process (6) has a problem in the treatment of waste water from the alkali fusion.
In the process (7), the selectivity of the dimerization reaction is so low that this process can hardly be practised unless byproduced phenol is used effectively.
The above-described processes which have heretofore been proposed as preparation processes for 4,4'-dihydroxybiphenyl are accompanied by their own drawbacks as mentioned above. A great deal of efforts has been exercised with a view toward making improvements thereto. No significant effects have however been achieved. Accordingly, the high price of 4,4'-dihydroxybiphenyl still remains as an obstacle for the expansion of its application field.